Recently got a nonworking ZX81 personal computer. On power on only white and black vertical stripes were visible. Having some experience with ZX Spectrum`s my call was that RAM is faulty. ZX81 uses 2114 ram chips. I have ordered coupke of Toshiba TMM314AP 1024 word x 4-bit high speed read-write memories operated with 5 V single power supply. The memories are static in oeration and require no clocks or refresh period. At least that what they say in a datasheet 🙂
My modification were:
Replaced all electrolytic capacitors, just in case.
Removed ram integrated circuits and added 18 pin sockets ( I did not have exact pin number sockets at my hand so just used longer ones that I have cut to length ) and inserted new RAM chips.
Made a composite mod using 2N3904 transistor and 100ohm and 50ohm resistors. Actually that one I made pretty neatly, proud of myself 🙂